TechnitiumSoftware/DnsServer · error · DnsServerException

DNS application already exists: {applicationName}

Error message

DNS application already exists: {applicationName}

What it means

Thrown by InstallApplicationAsync when an entry for the given applicationName already exists in the in-memory _applications dictionary. The DNS server treats each installed application (DNS App) as uniquely named; a duplicate name is rejected before any filesystem write so the existing install is not corrupted.

Source

Thrown at DnsServerCore/Dns/Applications/DnsApplicationManager.cs:375

                    }
                }));
            }

            await Task.WhenAll(tasks);

            RefreshAppObjectLists();
        }

        public async Task<DnsApplication> InstallApplicationAsync(string applicationName, Stream appZipStream)
        {
            foreach (char invalidChar in Path.GetInvalidFileNameChars())
            {
                if (applicationName.Contains(invalidChar))
                    throw new DnsServerException("The application name contains an invalid character: " + invalidChar);
            }

            if (_applications.ContainsKey(applicationName))
                throw new DnsServerException("DNS application already exists: " + applicationName);

            string applicationFolder = Path.GetFullPath(Path.Combine(_appsPath, applicationName));
            if (!applicationFolder.StartsWith(_appsPath + Path.DirectorySeparatorChar))
                throw new DnsServerException("The application name is invalid: " + applicationName);

            if (Directory.Exists(applicationFolder))
                Directory.Delete(applicationFolder, true);

            Directory.CreateDirectory(applicationFolder);

            //keep a copy of the zip file in the application folder for transferring to other nodes
            await using (FileStream zipCopyStream = new FileStream(Path.Combine(applicationFolder, applicationName + ".zip"), FileMode.Create, FileAccess.ReadWrite))
            {
                await appZipStream.CopyToAsync(zipCopyStream);

                zipCopyStream.Position = 0;

                await using (ZipArchive appZip = new ZipArchive(zipCopyStream, ZipArchiveMode.Read, false, Encoding.UTF8))

View on GitHub (pinned to d0484b6c1e)

Solutions

1. If you want to replace the app, call UpdateApplicationAsync(name, zipStream) instead of InstallApplicationAsync.
2. Call UninstallApplicationAsync(name) first, then InstallApplicationAsync.
3. Check DnsApplicationManager (or the apps API listing) before installing and skip when the name is already present.
4. Restart the server if you suspect _applications is stale relative to disk (e.g. directory was manually removed).

Example fix

// before
await appManager.InstallApplicationAsync(name, zip);

// after
if (appManager.ListApplications().Any(a => a.Name == name))
    await appManager.UpdateApplicationAsync(name, zip);
else
    await appManager.InstallApplicationAsync(name, zip);

bool AppExists(IApplicationManager mgr, string name) => mgr.ListApplications().Any(a => a.Name == name);
// call: if (AppExists(mgr, name)) await mgr.UpdateApplicationAsync(name, zip); else await mgr.InstallApplicationAsync(name, zip);

try { await mgr.InstallApplicationAsync(name, zip); }
catch (DnsServerException ex) when (ex.Message.Contains("already exists")) { await mgr.UpdateApplicationAsync(name, zip); }

When it happens

Trigger: Calling InstallApplicationAsync(name, zipStream) twice with the same name without first calling UninstallApplicationAsync. Or an automated/HA node re-running install after the dictionary was populated from a previously synced app folder on startup.

Common situations: Re-uploading a zip through the web UI thinking it will 'upgrade'; scripting node-to-node app replication that calls install instead of update; a half-uninstalled app whose directory was deleted but whose entry was retained (rare).

Related errors

• Failed to update web server config. Please try again.
• User already exists: {username}
• User already exists: {newUsername}
• Group already exists: {name}
• Group already exists: {newGroupName}

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/458cac36032362ef. Report an issue: GitHub.